Excessive amounts of static electricity can easily and insidiously destroy or damage integrated circuit (IC) devices. Over-voltages or power densities resulting from static. electricity can destroy certain IC junctions and components by, among other things, radically altering the doping structure of the lattices or by impairing the silicon substrate of the IC. As a consequence, industries involved in the fabrication of ICs and other microelectronic components maintain such circuits and components and their environment at or near zero electrical potential during fabrication and packaging. Prevenative measures used in the industry include providing workers and work stations with anti-static carpet, using conductive or dissipative grounded desk top work surfaces, and providing workers with grounding straps such as that described in U.S. Pat. No. 4,577,256. Additionally, effective environmental control requires use of one or more hot air or other ion generators which emit ions to neutralize static charges.
Monitoring the air ion density in an area used for fabricating ICs also is an important factor in limiting or preventing component damage due to excess static charge. Commercially available monitoring devices include charge build-up monitors, which measure directly the static potential present in the area's atmosphere, and charge decay monitors, which measure the time needed for a given charge to be neutralized. Charge decay monitors according to the prior art use a mechanically chopped electrometer head with a vibrating reed to measure voltage on a single charge plate. The mechanical nature of the sampling device results in an increased possibility of system failure and precludes continuous sampling of the voltage on the plate.
Devices of the prior art also typically require that the user set a series of dials or switches in order to determine the starting and stopping voltages for the decay test. If the positions of any of these dials or switches are accidentally altered between tests, the resulting measurements from test to test will not compare. In addition, the large number of dials and swithes increases the likelihood that an incorrect setting on one or more will be used. As a consequence, many ICs may be damaged or destroyed before the mistake could be realized and the generators adjusted to produce sufficient quantities of ions.
One such prior art device, for example, contains a single 6".times.6".times.1/4" metal plate positioned so as to have a 20 pf capacitance between it and electrical ground and uses a mechanically chopped head as mentioned above. A 0-150 second timer also is provided for mesuuring the decay time of the voltage on the plate. This device allows the user to start the test at either 5 kV or 1 kV and stop it at 500 V or 100 V, respectively, or when the plate voltage crosses a manually selected cut-off value. A second such devce contains a microprocessor system so that the user may select from any variety of values the start and stop voltages to be used in the decay test. A liquid crystal display (LCD) provides alpha-numeric and graphical output of the results.